notif: Use live value for app ID on registering APNs token

[gnprice]

In my draft for #321 I have a TODO like this:

      case TargetPlatform.iOS:
        const appBundleId = 'com.zulip.flutter'; // TODO find actual value live
        await registerApnsToken(connection, token: token, appid: appBundleId);

This will be just fine as long as the app ID is indeed com.zulip.flutter. That means it will basically be fine right through the beta period, until we start rolling this app out as the new version of org.zulip.Zulip (the app ID currently occupied by the zulip-mobile RN app).

The fix is easy, locally:

        final appBundleId = (await PackageInfo.fromPlatform()).packageName;

The one wrinkle is that that version breaks tests, because it's directly calling a plugin. To fix it, we'll want to indirect PackageInfo.fromPlatform through a method on ZulipBinding, like we've done for other plugins. This still isn't hard, but given the impending beta, I'm deferring it for now.

[Abhisheksainii]

My approach to solve the issue:

1. Update ZulipBinding to provide package info: Add a method in ZulipBinding to return the package name. This method will wrap the call to PackageInfo.fromPlatform() and make it test-friendly.

2. Override the method in TestZulipBinding: In the TestZulipBinding, override getAppBundleId to return a mock or predefined value for testing.

3. Refactor the code to use getAppBundleId: Replace direct calls to PackageInfo.fromPlatform() with the new getAppBundleId method from ZulipBinding. Like this (final appBundleId = await ZulipBinding.instance.getAppBundleId();).

4. Write a test for the above logic.

[Abhisheksainii]

I have started working on the issue and would like to claim it.
I will be making changes to the Zulip binding class and its corresponding test class. Adding packageInfo wrapper for bundleID.
Plus will be adding test for the same.